ci: remove staging promotion pipeline

[serrrfirat]

Summary

• delete the staging promotion workflows and helper scripts
• remove the release-plz batch-summary flow that depended on staging promotion merge markers
• simplify docker.yml by removing the scheduled staging image path and retiring the :staging override

Notes

• This PR is stacked on top of ci: prepare workflows for GitHub merge queue #2779.
• For staging deployment targeting, we are treating staging as tracking main directly rather than introducing a new deploy/staging tag workflow.

Validation

• ran git diff --check
• parsed the edited workflow YAML locally
• did a focused read-only review pass on the final diff

Part of #2719.

[gemini-code-assist]

Code Review

This pull request removes two bash scripts used for updating pull request bodies with staging promotion summaries and metadata. I have no feedback to provide as there are no review comments to evaluate.

[henrypark133]

What looks good:

• The retired staging-promotion workflows and helper scripts are removed cleanly.
• docker.yml now explicitly rejects staging tag overrides, which matches the stated direction.
• I did not find stale references inside this PR's actual six-file diff.

No verified findings.

Low-priority notes:

• This PR is stacked on issue-2719-pr-a-merge-queue-ready, so rollout risk around branch triggers and required checks lives mostly in the base branch rather than in this cleanup diff itself.

Summary:

• Recommended verdict: Comment
• Prior feedback status: no prior review
• Review coverage: checked-out worktree plus CI-focused diff audit
• Residual risk: evaluate the full stack together before merge, since origin/HEAD still points to origin/staging

[think-in-universe]

Do we plan to completely remove the staging tag from ironclaw docker image?

I think we probably still want to keep a staging or dev tag for tracking the build from the latest commit in main branch even if we're replacing staging branch with main.

[henrypark133]

Closing this as superseded by #3104. The staging cleanup is folded into the main-only cutover PR so the workflow deletion and merge-queue CI contract land together from current main.